My linux root OS device is
/dev/sda, and my external hard drive is
/dev/sdb (empty drive)
I don't want to loss data on
/dev/sda, and setup raid level 0.
Is this possible?
With RAID 0, your data is split in half (evens and odds) between the drives. In other words, there are chunks 1, 3, 5, .... etc. on the first drive, and a second group of chunks 2, 4, 6, etc. are on the second drive. If one of the drives dies, you've instantly lost 50% of the chunks. Imagine opening up your program and deleting every other line out of it. That's what happens when you lose a striped disk. Depending on the stripe size, you may be able to recover some data or even whole files out of the remaining disk (it's very possible that a 10KB file would be completely intact on one disk since your blocks should be larger than that). However, a file that's ten times your block size would have 5 blocks on each disk, meaning you just lost 50% of your file.
Important to note that you can't set up RAID 0 without formatting (initializing) the drives as RAID 0. You would have to back-up your data before creating the raid.